The present invention suggests efficient distillation schemes to separate multicomponent mixtures containing three or more components into product streams each enriched in one of the components. Generally, the objective of a process engineer designing a distillation scheme is to make it more efficient by reducing the heat requirement of the distillation columns within the distillation scheme. The distillation schemes known in the literature that require lower heat duty are quite complex and difficult to operate. As a result, many of these schemes lack operating flexibility and are rarely used in industry. Therefore, there is a need for distillation schemes that are not complex while having low heat requirements. The present invention is an answer to this long desired need for improving the operating flexibility of multicomponent distillation while maintaining lower heat duties.
Consider the separation of a ternary mixture having components A, B and C (mixture ABC) into three product streams each enriched in one of the components. A is the most volatile component and C is the least volatile component. To separate a ternary mixture ABC into almost pure components it is required that a distillation scheme use two distillation columns. Such distillation schemes are well known in the art. There are such five well known schemes: direct sequence, indirect sequence, side rectifier, side stripper and thermally coupled columns. Each of the schemes will now be described in detail:
FIG. 1 shows a direct sequence scheme. A feed mixture containing components A, B and C (stream 10) is fed to first distillation column 100 having a condenser 110 and a reboiler 120 where it is distilled to A-enriched product (stream 70) from the top. The liquid from the bottom of this column (stream 20) is primarily a binary mixture composed of components B and C. This BC liquid stream is split into two streams. A first portion (stream 22) is fed to second distillation column 200. The second portion (stream 24) is boiled and fed as stream 26 to the bottom of first distillation column 100. B-enriched product (stream 80) and C-enriched product (stream 90) are produced from second distillation column 200 having a condenser 210 and a reboiler 220. A portion of the C-enriched bottoms liquid is boiled (stream 92) and returned to the column to provide boil-up. It is noticed that the only mass transfer between the two columns is the transfer of stream 22 from first distillation column 100 to second distillation column 200. There is no return stream from second distillation column 200 to first distillation column 100. This is an example of only a one-way communication between the two columns.
FIG. 2 shows an indirect sequence scheme. Distillation of the feed mixture (stream 10) in first distillation column 100 having a condenser 111 and a reboiler 121 produces C-enriched product (stream 90) from the bottom and a primarily binary vapor mixture AB (stream 30) from the top. A portion of this saturated vapor stream is fed to second distillation column 200 (stream 32). Another portion (stream 34) is condensed and sent as reflux (stream 36) to first distillation column 100. A-enriched product and B-enriched product (streams 70 and 80 respectively) are produced from second distillation column 200 having a condenser 211 and a reboiler 221. Once again there is only one one-way communication through stream 32 between the first and second distillation columns.
FIG. 3 shows a side rectifier scheme wherein the feed mixture ABC (stream 10) is distilled in first distillation column 100 having a condenser 110 and a reboiler 121 to produce A-enriched product (stream 70) from the top and C-enriched product (stream 90) from the bottom. A portion of the C-enriched bottoms liquid is boiled (stream 92) and returned to the column to provide boil-up. The component of intermediate volatility, B, is collected (stream 80) from the top of second distillation column 200 (also known as a side rectifier) having a condenser 210. It is important to notice that second distillation column 200 does not have a reboiler at the bottom and instead it is fed by a vapor (stream 50) which is withdrawn from a location below the feed of first distillation column 100. This vapor stream is primarily a binary mixture consisting of components B and C. The liquid (stream 52) from the bottom of second distillation column 200 is sent to first distillation column 100 at the same location as where the vapor (stream 50) was removed from first distillation column 100. This leads to a two-way communication between the two distillation columns. In a two-way communication mode, when a vapor stream is sent from one column to another column, then a return liquid stream is implemented between the same locations of the two columns. As compared to the schemes in FIGS. 1 and 2, the number of reboilers is reduced by one, and the total number of reboilers and condensers used are three vs. four.
FIG. 4 shows a side stripper scheme which is similar to FIG. 3 (corresponding streams and equipment use the same identification) except that the feed to second distillation column 200 (now known as side stripper) is a liquid (stream 60), product B (stream 80) is collected at the bottom of second distillation column 200 instead of the top and second distillation column 200 has a reboiler 221 but no condenser. The liquid stream is withdrawn from first distillation column 100 from a location which is above the feed location to first distillation column 100 and is primarily a binary mixture composed of components A and B and is fed to the top of second distillation column 200. The vapor (62) from the top of second distillation column 200 is returned to first distillation column 100 resulting in a two-way communication between the two columns. In this two-way communication mode, a liquid stream is sent from first. distillation column 100 to another column then a return vapor stream is implemented between the same locations of the two columns. It is important to notice that as compared to the scheme in FIG. 2, one less condenser is used in FIG. 4.
FIG. 5 shows a scheme which uses two two-way communications between the first and second columns, thereby using only one reboiler 222 and one condenser 212 in second distillation column 200. The two-way communication at the bottom of first distillation column 100 is the same as the one shown in FIG. 3 and at the top is the same as the one shown in FIG. 4. B-enriched product (stream 80) is collected from an intermediate location of second distillation column 200. It is important to notice that due to two two-way communications, the total number of reboilers and condensers is reduced by two.
It is known that the schemes with two-way communication (shown in FIGS. 3-5) require less heat input than the ones without two-way communication (FIGS. 1 and 2) ("Minimum Energy Requirements of Thermally Coupled Distillation Systems", Z. T. Fidkowski and L. Krolikowski, AlChE Journal, pages 643-653, volume 33, 1987). The heat requirement in reboiler 121 in FIG. 3 is less than the total heat requirement in both reboilers of FIG. 1. Similarly, total heat input in the reboilers for the side stripper configuration in FIG. 4 is less than the total heat input in the reboilers of indirect sequence in FIG. 2. However, several studies have proven that for the separation of ternary feed mixtures into pure product streams, the configuration with two two-way communication in FIG. 5 requires the least heat input in its reboiler. It has been said that, on average, the configuration with two two-way communications requires 30 to 50% less heat than the corresponding conventional arrangements of FIGS. 1 and 2; generally it also requires much less heat than the configuration with only one two-way communication shown in FIGS. 3 and 4. It is taught in the prior art that two two-way communications are needed to achieve the lowest heat demand for ternary distillation.
While the heat demand for the scheme in FIG. 5 with two two-way communications is lowest, it has rarely been used. The lack of use has often been attributed to fear of control problems ("Thermal Coupling for Energy Efficiency", H. Rudd, Supplement to the Chemical Engineer, pages S14-S15, Aug. 27, 1992; "The Design and Optimization of Fully Thermally Coupled Distillation Columns", C. Triantafyllou and R. Smith, Trans. IChemE, pages 118-132, Volume 70(A), 1992). One of the often cited concern is the flexibility to control the flows over a wide range both at the top and bottom ends of first distillation column 100. For the vapor AB in stream 32 to flow from first distillation column 100 to second distillation column 200 it is required that the pressure at the top of the first column be greater than the pressure at the feed point of stream 32 in second distillation column 200. At the same time, for the vapor BC in stream 27 to flow from second distillation column 200 to first distillation column 100 it is necessary that the pressure at the bottom of first distillation column 100 be lower than the pressure at the originating point of stream 27 in the second column. This requires careful adjustment of pressure in both the columns. Furthermore, both the liquid transfer streams 37 and 22 at the top and bottom of first distillation column 100 flow in a direction opposite to the vapor flow at each end. This requires that either a pump be used on each of the liquid streams or relative height of the two columns be adjusted to allow each of the liquid stream to be transferred through gravity. All these factors impose constraint on the operating flexibility of the scheme with two two-way communication. Clearly, there is a need for alternative solutions with higher operating flexibility while maintaining lower heat demand for distillation.
The same challenge exists when mixtures containing more than three components are distilled to produce product streams each enriched in one of the components. The reason being that the distillation schemes with low heat demand used to distill mixtures with more than three components are made up of the ternary subscheme shown in FIG. 5. Therefore deficiencies of the ternary subscheme are also carried to the distillation of mixtures containing a greater number of components. Some known examples of four and five component distillation schemes can be found in a paper by Agrawal ("Synthesis of Distillation Column Configurations for a Multicomponent Separation", Ind. Eng. Chem. Res., volume 35, pages 1059-1071, 1996).